In aerospace systems, such as engine exhaust ducts, nose cones, firewalls, and reentry shield surfaces, surfaces may be exposed to high temperatures or large temperature gradients and must, accordingly, be insulated. Each application has unique problems which have rendered it difficult to provide an adequate thermal insulation that can be tailored for optimum performance in that application.
Recently, low-density ceramic fibers have been used for insulating aerospace surfaces. For example, the space shuttle's exterior surface is insulated with a plurality of ceramic tiles that are arranged in a closely spaced, ordered array. To provide the required fit, each tile is cut precisely from a fused ceramic blank. To form the blanks, silica fibers and other ceramic components were initially mixed into a slurry and cast into blocks. After drying, the blocks were sintered to form strong ceramic bonds between the overlapping fibers. The blocks were cut into smaller blanks that were subsequently milled into the final tiles. Each tile was bonded to an isolation pad with a high-temperature adhesive. The pad was, then, bonded to the underlying metallic substructure of the shuttle.
Especially during takeoff and reentry, a differential surface temperature distribution exists over the surface of the space shuttle. The fused ceramic tiles are vulnerable to shear forces caused by the differential surface temperature distribution. To prevent breakage, each tile must be small (generally less than ten inches on a side) thereby creating enormous fabrication and assembly costs.
Glass coatings have been developed to improve thermal shock resistance for ceramics. In U.S. Pat. No. 4,093,771, Goldstein discloses a borosilicate glass coating that is used on the surface of reusable silica insulation. In U.S. Pat. No. 4,381,333, Stewart discloses a two-layer glass coating for silica insulation. The base layer has a high emittance and is preferably formed by combining a reactive borosilicate glass with an emittance agent, such as silicon tetraboride, silicon hexaboride, boron, or silicon carbide. The outer layer is formed from discrete, sintered glass particles to provide a high scattering coefficient. Preferably, fused silica or a reactive borosilicate glass having a higher silica content than the base layer is used for the outer layer. In either the Goldstein or Stewart ceramics, the coating is sprayed onto the underlying fiber insulation before firing to form a glass.
Insulation may be formed with an unsolidified silica glass felt sandwiched between silica glass fiber cloth. The three layers are stitched together with silica glass thread (or another suitable refractory thread) and are bonded with adhesive to the surface to be protected. Similarly, a layering effect may be achieved by superposing a stitched blanket of silica and aluminoborosilicate fibers (commercially available under the trademark NEXTEL from 3M Company) over a separate, stitched blanket of silica fibers. By staggering the blankets and using suitable emittance coatings on the outer surfaces of the blankets, control of the insulative characteristics can be achieved, thereby countering the temperature distribution on and gradient through the insulation.
Lightweight fibrous insulation that permits a wide range of design choices in terms of insulative characteristics, strength, and durability is still needed.
Fiberformed ceramic insulation with surprising physical properties is described in U.S. Ser. No. 698,496, and is made by forming a slurry of ceramic fibers, molding the slurry to form a soft felt mat, drying the mat, and incrementally introducing a sol-gel glass binder into the mat to form a rigid mat. The incremental addition of the sol-gel binder is accomplished through a unique multiple impregnation technique in which a small amount of binder is initially impregnated into the mat, is gelled, and is cured to stabilize the mat dimensionally, allowing handling and further processing of the mat. The mat is strengthened thereafter to its final strength by successive additions of glass binder. This technique cures the mat to a rigid, predetermined shape without appreciable shrinking of the resultant structure, and is contrasted with prior processes in which the entire binder is introduced either in one impregnation of the mat or by incorporating the binder in the fiber-containing slurry prior to the molding or felting operation. U.S. Pat. No. 3.702,279 to Ardary et al. and U.S. Pat. No. 3,935,060 to Blome et al. exemplify these prior processes.
By forming a plurality of slurries containing different ceramic fibers, and molding each of the slurries in succession to form a single felt mat having interlocking layers of fibers, the thermal and mechanical characteristics of the resulting insulation can be controlled over a wide range. By using longer fibers, the insulation can be strengthened where desired, as for example, in the region that is joined to the skin or substructure Where a particular application requires further resistance to high temperatures or to large temperature gradients, a layer of high emittance can be formed at desired locations within the continuous fiber mat by including an emittance agent, such as boron or silicon carbide, in one or more slurry. A desired insulative profile can also be obtained by using ceramic fibers of different materials in the different slurries, thereby countering the effect of the temperature distribution on the surface. With these controllable variables and others, insulation can be made for a wide range of applications.
A vacuum-felting process tends to align the ceramic fibers parallel to the forming surface, producing an anisotropic material having reduced flatwise tensile strength. This anisotropic material can be mechanically strengthened by stitching the mat with glass or other high-temperature refractory thread in a direction that is normal or at angles to the mat fibers. If the mat has layers, the stitching provides additional connection between the layers. Stitching can also be used to anchor the glass fabric of the coating to the mat.
The fiberformed insulation usually includes a network of ceramic fibers that are disposed in a plurality of layers, with fibers within each layer intersecting other fibers within the same layer. Some fibers within each layer intersect fibers in adjacent layers. To strengthen the layered network, sol-gel glass bonds are formed where the ceramic fibers intersect.